I'm not a fan of the 'work a 2nd job' thing. I think you're usually better off either developing your skills in your primary trade or working on some other aspect of your life/finances than taking on a 2nd job, at which you will be taxed at your marginal rate.
The exception would be where you're a permanent employee at your first job and you moonlight to do some cash in hand work using sick leave from your first job (that way you don't do any 'extra' work). But this is a breach of contract and is risky for a number of reasons. Doable though if you're clever enough, and lucrative.
I personally think the 2nd job advice from people like Ramsey is partially to get people to realize how serious the situation is. Like "if you are in debt, yes, it really is serious enough to get a second job, so take it that seriously".
Also, for many people, just becoming more successful at their main career is quite the uphill battle. Most people would be more successful if they knew how to be. I'm not saying they can't learn how to be, but that can take time and be a significant barrier for many.
Taking on a second job can also really jolt someone out of complacency about their core career and income. And lastly, it can really push people to re evaluate the value of their time and money.
I know for me, I spent a half year working additional evenings and weekends early in my career, and making extra money at such an enormous personal cost really kicked my ass into valuing my time and not wasting the money that took time to make.
Overall, my point is that yes, it is more efficient and effective to just be more successful, but for the people that Ramsey is designed to help, that's unlikely to be a straightforward and timely option for them. It also doesn't fit nearly into a series of steps since the path to greater career success is highly individual.
It's hugely common advice here on the MMM forums though, since here it's all about personalized approaches to a similar philosophy.
Basically, different people need different advice at different stages of readiness.